1. Field of the Invention
The present invention relates to a configuration of a piezoelectric type inkjet printer head.
2. Related Art
Japanese Patent-Application Publication No. HEI-11-334064 discloses an example of conventional on-demand type piezoelectric inkjet printer head, which includes a head case and a head member fixed to the head case by an adhesive. The head member includes a substrate, energy generating members, and diaphragms. The substrate is formed with a plurality of nozzles, a plurality of pressure chambers corresponding to the nozzles, and a manifold for supplying ink to the pressure chambers. The energy generating members, such as piezoelectric elements, are one-to-one correspondence with the pressure chambers and attached on a surface of the substrate with the diaphragms interposed therebetween.
The head case is formed of electrically conductive resin and detachably supports ink cartridges. A conductive coating material is applied over the side surfaces of the head casing and the head member to form a conductive layer thereon. The conductive layer is electrically connected to a carriage shaft via an earth plate.
In this configuration, metal components are not charged by static electricity which may be generated when a recording sheet contacts the head member, thereby preventing breakage of the head member due to the static electricity. Also a metal cover for covering over a nozzle surface of the head member is dispensed with. This shortens the distance between the nozzles and the recording sheet, resulting in higher printing quality.
However, providing the above head member requires a number of process steps. A common-electrode film is first formed on the diaphragm, and piezoelectric films are formed thereon at positions corresponding to the pressure chambers by patterning techniques. Then, a drive electrode is formed on each piezoelectric film. Moreover, there is only a single layer of piezoelectric film that deforms the diaphragm, providing only insufficient deforming amount of the diaphragm, so that effective ink ejection cannot be performed.
In order to overcome this problem, the present inventor has proposed in Japanese Patent-Application Publication No. 2000-258007 an inkjet printer head that includes a cavity unit and a piezoelectric actuator. The actuator has a laminated structure of piezoelectric ceramic sheets, common electrodes, and drive electrodes laminated such that each piezoelectric ceramic sheet is sandwiched between a common electrode and a plurality of drive electrodes. The piezoelectric ceramic sheet has a thin thickness of 20 xcexcm to 30 xcexcm. The cavity unit is formed of metal with pressure chambers. The bottom surface of the lowermost piezoelectric ceramic sheet is fixed by an adhesive layer to the cavity unit such that the drive electrodes formed on the upper surface of the piezoelectric ceramic sheets are in vertical alignment with the pressure chambers formed in the cavity plate.
However, in this configuration, an ejection voltage applied to the drive electrode on the lowermost piezoelectric sheet is adversely applied to the cavity unit also via the lowermost piezoelectric sheet, and also to soluble ink, i.e., conductive ink, contained in the pressure chambers. As a result, when an ejection voltage is applied to a drive electrode so as to eject ink from a corresponding pressure chamber, electric current conducts through the piezoelectric ceramic sheets, the cavity unit, and the ink to different drive electrode corresponding to an adjacent pressure chamber, thereby ejecting ink from the unintended adjacent pressure chamber.
Moreover, leakage of the ejection voltage applied to the drive electrode on the lowermost piezoelectric sheet delays the deforming timing of the lowermost piezoelectric sheet from that of the other piezoelectric sheets. Accordingly, deforming movement of the other piezoelectric sheets in response to the ejection voltage applies pressure to and deforms the lowermost piezoelectric sheet, and subsequently the lowermost piezoelectric sheet deforms spontaneously in response to the ejection voltage. This difference in the deforming timings affects pressure change in the pressure chamber, whereby ink ejection performance becomes unstable. Moreover, when lowermost piezoelectric ceramic sheet and/or the adhesive layer have an uneven thickness, the difference in the deforming timings varies even within the single lowermost piezoelectric sheet, further degrading ink ejection performance.
It should be noted that although the lowermost piezoelectric sheet sandwiched between the drive electrodes and the cavity unit rather than between the drive electrodes and the common electrode does not theoretically have active portions that spontaneously deform when an ejection voltage is applied to the drive electrodes formed thereon, the lowermost piezoelectric sheet is in fact polarized and thus deform spontaneously, which is for the existence of the ink in the pressure chamber and of the cavity unit.
It is an object of the present invention to overcome the above problems and to provide an inkjet printer head with a stable ink ejection performance.
In order to achieve the above and other objectives, there is provided an inkjet head including a cavity unit, an actuator, and a conductive member. The cavity unit is formed of a conductive material with a plurality of nozzles and a plurality of pressure chambers in fluid communication with the corresponding nozzles. The plurality of pressure chambers is aligned in a predetermined direction. The actuator includes a plurality of sheet members laminated one on the other in a lamination direction, a plurality of driving electrodes corresponding to the pressure chambers, and a plurality of common electrodes. Each sheet member has a width greater than a total width of the plurality of pressure chambers with respect to the predetermined direction. The plurality of driving electrodes and the plurality of common electrodes are arranged in alternation with respect to the lamination direction. Each of the driving electrodes and the common electrodes is sandwiched between corresponding sheet members. Portions of the sheet members sandwiched between the driving electrodes and the common electrodes serve as active portions that selectively eject ink droplets from the corresponding pressure chambers through the nozzles. The conductive member electrically connects the common electrodes and the cavity unit for maintaining the common electrodes at the same potential as the cavity unit. One of the sheet members has a first surface on which one of the common electrodes is formed and a second surface attached to the cavity plate.
There is also provided an inkjet head including a cavity unit, an actuator, and a conductive member. The cavity unit is formed of an electrically conductive material with a plurality of nozzles and a plurality of pressure chambers in one-to-one correspondence with the nozzles. The nozzles are aligned in a predetermined direction. The actuator is attached to the cavity unit and includes a plurality of sheets laminated one on the other and a plurality of drive electrodes positioned between corresponding sheets. The sheets have a width greater than a total width of the pressure chambers with respect to the predetermined direction. The drive electrodes correspond to the pressure chambers. The conductive member grounds the cavity unit. Plural ones of the drive electrodes located closest to the cavity unit confront the cavity unit with more than one of the sheets interposed between the plural ones of the drive electrodes and the cavity unit.